Miswired bathroom fan, light, heater needs 14/4+Gnd (not 14/3)

I encountered this thoroughly miswired switch location while attempting a ‘simple’ faulty GFCI receptacle replacement. The 20A GFCI circuit served the GFCI as well as a 3-in-one fan/light/heat control for the bathroom overhead fixture. Unfortunately, the 14/3 plus ground (black, red, white, and bare) cabling for the fixture was too small (#14 conductors rated for 15A, not 20A) and included only two phase (hot) conductors, whereas the three switched loads integrated into the fixture require three hot conductors.

 

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The solution? The real trouble with the wiring is that the installer used the white wire for phase (hot), switched for the light. Then, because there was now no grounded conductor (neutral) available, they used the bare equipment grounding safety wire (ground). A bare conductor should never carry normal current; only fault current for the purpose of tripping the circuit overcurrent device (breaker) [in guarded, supervised industrial applications, a bare conductor -- usually a bus bar -- may be a current-carrying conductor; in such situations, the area where the conductor runs is essentially a super-sized wiring enclosure big enough for people to walk into (after unlocking padlocks, and only with authorization from qualified personnel)].

 

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To fix the problem, I abandoned the heating function (and miswired circuit) for the ceiling fixture, restoring the bare wire to its function as safety ground wire, and the white (neutral) as grounded current-carrying conductor. Then, to limit current in the circuit to the capacity of the smallest conductors in it, I downsized the branch circuit breaker from 20A to 15A.

 

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The proper cable for this application would have been 12/4 + Gnd (black, red, blue for switched phase conductors; white for grounded current-carrying conductor, and bare for equipment safety grounding). Alternatively, the installer could have installed two 12/2 cables, using:

• CABLE A: black>lights.  white>grounded current-carrying conductor for all functions in the fixture. bare>common safety grounding.
• CABLE B: black>fan. white>marked red for heater. bare>common safety grounding.

However, this is not ideal because it requires marking of a conductor smaller than #6, for use other than as indicated by its insulator. And, the current in both conductors of ‘Cable B’ is flowing in the same direction (black and white alternate in phase with each other). The magnetic field generated by alternating current in ‘Cable A’ would mostly cancel out, since current on the grounded conductor moves opposite that of the switched phase conductor (although in this case, the grounded conductor may be carrying more current than the lighting phase conductor, since it is returning fan and heat current to ground, as well). A 12/4 cable keeps current in both directions a) close together, for optimum canceling of magnetic field, and b) balanced.